British Standard Cycle (BSC orBSCy orCEI) is a British Imperial screw thread standard. Unlike other major British imperial thread standards (British Standard Whitworth andBritish Standard Fine) the thread runs at a 60degrees rather than a 55 degreesangle. All sizes1⁄4 inch and larger use 26threads per inch (tpi), making them similar to 1 mmISO threads, which are 25.4 per inch and also run at a 60 degrees angle. It was originally used with bothbicycles andmotorcycles.[1] However it is now believed[by whom?] to be obsolete in motorcycle manufacture. In the bicycle industry it is still found on virtually all bottom bracket threads and the wheel axles of low-end models manufactured in China, which are derived from pre-WWII British roadsters.[citation needed]
BS 811: 1950 provides specifications for British standard cycle threads.[2]
Cycle thread in7⁄16 and1⁄2 inch sizes also come in 20 tpi and 24 tpi options.
1/4" diameter cycle thread nuts and bolts have the same 26tpi as 1/4" BSF, which means that they will fit each other in this diameter only.
Traditionally the parts it would be found on are:
Note that M8 × 1.00 metric threaded parts can interchange with5⁄16 inch (7.94 mm) diameter × 26 tpi parts with a "Class B fit", meaning you should not go back and forth between threadings.
British Bicycle standards for other parts which are not strictly BSC but come in similar fine threads include:
The thread was defined to meet a requirement for a thread form forbicycles andmotorcycles, originally specified by theCycle Engineering Institute. Before being adopted as a British Standard it was known as theCEI thread.[citation needed]
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